In recent years, active efforts have been made to develop an electronic device (compound semiconductor device) provided with a GaN layer and an AlGaN layer formed on and above a substrate in this order to use the GaN layer as electron transit layer. One example of such a compound semiconductor device is a GaN-based high electron mobility transistor (HEMT). Using the GaN-based HEMT as a switch of a power supply inverter may achieve both the reduction of on-resistance and the improvement of withstand voltage. The HEMT is also capable of reducing standby power consumption and increasing operating frequencies, as compared with an Si-based transistor. Switching losses and the power consumption of an inverter may therefore be possible. In addition, as far as transistors of like performance are concerned, the GaN-based HEMT may be made smaller in size than a Si-based transistor.
In the GaN-based HEMT whose GaN layer is used as an electron transit layer and whose AlGaN layer is used as an electron supply layer, strain arises in AlGaN due to a difference in lattice constant between AlGaN and GaN. This causes piezoelectric polarization and a high-concentration two-dimensional electron gas (2DEG) is generated. Accordingly, this GaN-based HEMT is applied to a high-power device.
It is difficult, however, to manufacture a GaN substrate excellent in crystallinity. For this reason, a GaN layer, an AlGaN layer and other layers are conventionally formed on and above an Si substrate, a sapphire substrate or an SiC substrate by heteroepitaxy in most cases. The Si substrate, in particular, is easy to obtain at low cost as a large-diameter high-quality substrate. Accordingly, a structure with GaN and AlGaN layers grown on and above a Si substrate has been actively studied.
However, the GaN layer, the AlGaN layer and the Si substrate have significantly different thermal expansion coefficients. In addition, high-temperature treatment is requested for the epitaxy of the GaN and AlGaN layers. Accordingly, warpage, cracks or the like may occur in the Si substrate due to the different thermal expansion coefficients at the time of such a high-temperature treatment. In view of such a problem associated with the different thermal expansion coefficients, there has been proposed a composite substrate in which Si is crystal-grown on a sapphire substrate.
It is difficult, however, to grow an excellent Si crystal on a sapphire substrate. In addition, sapphire and Si have a larger difference in thermal expansion coefficient than that between a nitride semiconductor and Si. Thus, warpage, cracks or the like is more likely to occur in the composite substrate. This is also true for a composite substrate in which an Si substrate is bonded to a sapphire substrate.    [Document 1] Japanese Laid-open Patent Publication No. 2005-235989    [Document 2] Japanese Laid-open Patent Publication No. 11-214798    [Document 3] Japanese Patent No. 4126863    [Document 4] Japanese Laid-open Patent Publication No. 2010-161359